Clicking the link will explain what a DI is and does. As to uses, many will run some instruments through a DI instead of micing the amp, often to save space on a tight stage. Using them to input electronic devices that are line level to take them down to mic level, keyboards, computors and other sound boards or effects racks that are onstage.
Others that more knowledgable in sound will hopefully add more or clarify what I have said.
Can you not also use them to take a unbalanced signal and balance it? Like if you are running your main output from something that either only has 1/4" or RCA and convert it to XLR to reduce your interference as you run it to your speakers.
